|For activists, the 31st of each month marks the right to free assembly guaranteed in Russia's constitution [Reuters]
Russian police have arrested more than 100 people, including Boris Nemtsov, a former deputy prime minister, who were taking part in anti-Kremlin protests in Moscow and St Petersburg.
Opposition leaders and rights activists have been converging at the capital's Triumph Square on the 31st of each month, symbolising the right to free assembly guaranteed in Article 31 of Russia's constitution.
Authorities have justified crackdowns on previous protests at the square by saying the demonstrators lacked official permission.
Tuesday's arrests came a day after Vladimir Putin, Russia's prime minister, announced that any demonstrators without permits could expect harsh treatment.
About 70 people were detained in Moscow while 50 others were detained in St Petersburg, Viktor Biryukov, a police spokesman said on Tuesday.
Putin had defended police crackdowns on pro-democracy protesters in an interview published on Monday.
"Go without permission, and you will be hit on the head with batons. That's all there is to it," he said.
The activists say the constitutional protection of free assembly means prior permission is not required.
'Russia without Putin'
Protesters had gathered in Triumph Square calling for their right to freedom of assembly to be respected and shouting "Shame" and "Russia without Putin".
Lyudmila Lyubomudrova, 64, raised a banner reading: "Free the Constitution, Save Russia."
She told the Reuters news agency: "I am not afraid. Why should I be afraid? It's my country."
Many of the demonstrators were dragged on to buses by police officers.
Among the activists taken into custody were Eduard Limonov, an opposition party leader, Konstantin Kosyakin, a Left Front opposition movement leader and Nemtsov, who is now the co-chairman of the Solidarity opposition movement.
Moscow authorities had declared Triumph Square off-limits for demonstrations and closed off its centre last week to make way for construction of an underground parking garage.
Opposition groups said the abrupt closure was a pretext to stop the protests.
EU, US criticism
The detentions have drawn criticism from the United States and European governments.
"There are hundreds of police and to us it looks disproportionate," Thijs Berman, a Dutch member of the European Parliament's human rights subcommittee, said at Triumph Square.
"It begs the question: what is such a big country as Russia afraid of?"
Ileana Ros-Lehtinen, a US Republican representative on the House Foreign Affairs Committee, said Russia "must be held accountable for its crackdown on all forms of dissent, including the murders of journalists".
"Responsible nations cannot overlook Russia's downward spiral towards tyranny and oppression, and must deny Russia membership in the World Trade Organization and all of the other perks which it does not deserve," he said.
Dmitry Medvedev, the Russian president, has said the development of civil society and the rule of law is crucial for Russia's future.
But activists say police conduct and restrictions on protests show the Kremlin is determined to silence critics.